This invention relates to improved well logging methods and apparatus, and more particularly relates to novel methods and apparatus for providing a plurality of functionally integrated, depth-correlated subsurface measurements.
It is well known that oil and gas are found in subsurface earth formations, and that wells are drilled into these formations to recover such substances. However, it is usually necessary to survey or "log" the entire length of the borehole to ascertain if any of the formations contain significant recoverable amounts of oil and gas to justify completing the well.
In the early days of oil and gas exploration, wells were not extremely deep and information relating to the physical parameter of the subsurface formations was not complex. Accordingly, well logging was performed by a logging "tool" or sonde which was merely suspended at the bottom of the borehole at the end of a cable, and was then raised through the borehole as it generated measurements of one or more earth parameters. Circuitry was usually provided in the tool for converting such measurements into appropriate electrical signals which, in turn, were transmitted to the surface by one or more electrical conductors within the logging cable.
Over the years, however, these early deposits became depleted and with the continued search for oil and gas, wells became ever deeper and more expensive, requiring an increase in the sophistication of drilling techniques and improved knowledge with increasing detail reliability about the subsurface formations through which the well passed. In more recent years, oil and gas have become increasingly scarce with a corresponding increase in value. These factors have led to secondary and even tertiary-recovery projects which require even more detailed knowledge of the subsurface formations and, in particular, the fluids contained therein.
As wells became deeper, logging cables became longer and correspondingly hotter, their losses at both high and low frequencies becoming more severe. As a result, electrical signals containing information relating to the physical characteristics of the subsurface formations which have been developed by logging instruments became attenuated and distorted, reducing the accuracy of the information obtained. Further, limited cable band width has caused problems even when pulse/digital techniques are employed utilizing digital encoding of the measurement data in the tool and prior to transmission to the surface.
To compound the above-described problems, new logging instruments were being provided which had the ability to measure a plurality of physical parameters. Additionally, the practice developed whereby more than one tool was placed in a common chassis. For example, it is well known in the prior art to provide a logging tool having up to three data sources which may also be required to monitor parameters indicative of proper tool operation, such as temperature or other similar parameters which require an increased need for handling multiple data sources.
The above-described data recovery problems are compounded by the requirement for locating thin formation zones containing oil and gas. However, there is no single well logging technique or device which can provide a direct indication and evaluation of oil or gas in a particular formation of interest. Instead, a variety of logging techniques have been devised, which measure various different physical parameters of the earth substances adjacent the borehole, whereby such information can then be used according to selected functional relationships to determine those formations of probable or possible value.
For example, it will be readily apparent that if the oil and gas are diffused or dispersed in the cavities between the pore spaces within a formation, then a formation of greater porosity will more likely contain significant recoverable amounts of oil and gas than will a formation of lesser porosity. Accordingly, techniques and apparatus for deriving an indication of the relative porosities of the earth materials along the borehole will obviously be of value in determining the depths as which oil and gas will most likely be found in commercial quantities.
Also of value are techniques and devices used to measure the electrical resistivity of the earth substances along the borehole, and other devices and techniques used to measure the travel time or velocity of an acoustic pulse moving through such materials. In such case, the measurements are usually generated in the form of a current or voltage representative of the earth parameters being surveyed.
Another type of logging technique involves measurement of nuclear radiation occurring within a subsurface formation. The radiation may be naturally occurring or created by bombarding the interior of the borehole with radiations such as neutrons or gamma rays which thereafter engage in various interactions with the nuclei of the formation materials. Measurements are accordingly made of radiations which enter the well naturally or as a result of bombardment, and which may then be counted to provide indications of various earth parameters of interest. More particularly, the resulting radiations of interest may be sensed by a scintillation counter or the like, which generates electrical pulses as a function of those radiations detected, and these pulses may be then counted either at the surface or by suitable circuitry in the logging tool.
Since no one earth parameter can of itself provide a definitive and conclusive indication of the presence of oil and gas in commercial quantities, there has been a continuing need to perform as many different types of logging measurements as possible.
As logging tools become more sophisticated, such as those employing neutron generators capable of being rapidly pusled on and off, the problem of handling multiple data sources increases. Accordingly, the prior art contains neutron-lifetime tools, porosity tools, induction tools, resistivity tools, chlorine logs, shale indicators, carbon oxygen and calcium silicon logs, and any number of other specialized tools. However, no single tool will perform more than a few of these functions whereas in a single well, many such parameters are important. The failure to measure some of them has in the past led to an incorrect evaluation of the physical parameter of the subsurface formations. As a result, prior art techniques have provided data which cannot be clearly and reliably interpreted in the absence of other different but functionally correlative measurements. This, in turn, has also contributed to the need to provide logging instruments and systems for generating a plurality of different logging measurements, whereby the array of such measurements will be more informative as to the character of the earth materials of interest.
However, instruments cannot merely be attached, end on, to provide the increase in logging measurement information. Physical and operational constraint require careful positioning of the various instruments making up a multi-instrument tool. For example, the physical strength of the mandrel material used to form some instruments will not support the weight of other instruments. Additionally, operational constraints must be considered in placement of the instruments within the tool so that operation of the particular tool will not interfere with the measurements obtained by the other instruments therein. Further, some instruments, particularly those measuring nuclear radiation, can be calibrated as a unit and should be connected as such.
Unfortunately, as the number of different logging measurements generated by a single tool increases, so does the difficulty in recovering the measurement signal in both analog and digital forms. Most instruments are free running, i.e., they continually generate measurement data signals for transmission. As above-mentioned, when the measurement data signals are transmitted to the surface, various problems may occur which degrade the signals causing an attendant loss in data accruing. Cross talk is such a problem which may occur between conductors in the logging cable, degrading the measurement signals. Another problem is that different instruments require different time sequences for providing meaningful measurements. In particular, where the instruments are generating free reunning measurements as above-mentioned, the various measurement signals recovered at the surface can only be synchronized approximately and with difficulty. Still, another problem is some instruments utilize up to five conductors for their operation. For example, one type of logging technique utilizes an instrument which with two power conductors, a transmitter select conductor, a receiver select conductor and a signal conductor for transmission of data to the surface. Thus, it may be seen that use of this instrument even with a standard seven conductor logging cable precludes the use of a number of other instruments in combination.
To add to the above-mentioned problems, in recent years it has become a matter of extreme importance to conduct logging operations in deep wells with a minimum number of separate traversals of the borehole. This requirement has developed for a number of reasons. One reason is the fact that when deep holes are drilled a very large and expensive drilling rig is required, and this rig must remain in position at the well site but inoperative during the logging operation. Thus each hour required for logging may be counted as expense which may amount to hundreds or even thousands of dollars essentially wasted.
Another reason for avoiding lengthy and protracted logging procedures is the risk that the newly drilled hole may collapse or cave in or otherwise be damaged such that remedial work or even redrilling of the hole might be required. Further, it has been found that numerous traversals of the logging cable through the borehole may result in damage to the casing in the upper or intermediate portion of the hole where casing is ordinarily installed preparatory to drilling the lower portion of the well where potential hydrocarbon productive zones are sought or expected. In successively traversing the well, the hardened steel sheath of the logging cable may slice into the casing with consequent damage thereto.
Additionally, it is important that the logging measurements be acquired as soon as possible after drilling is completed in order that there be minimum undesired effects due to progressive invasion of drilling mud filtrate into permeable formations. Such filtrate invasion renders the detection of hydrocarbons more difficult and less accurate with the passage of time. Also, it is desirable to acquire all the logging measurements under the same conditions of borehole and formation temperature, as will be true if all measurements are taken at the same time.
However, even more important than the above-mentioned reasons is that each of the several logs need to be accurately correlatable with every other log with respect to depth in the borehole. Unfortunately, since the logging cable is elastic, variations occur which make such correlation difficult between instruments connected in a single chassis and utilized for a single run, not to mention the difficulties encountered in correlating. Nonetheless there is substantial advantage in making different logging runs made in multiple passes.
Customarily, as each logging measurement is obtained, an approximation of the depth at which the measurement is obtained is simultaneously derived and correlated to the measurement in a time relationship. However, when tools capable of generating different measurements during the same run are used, the depth approximation must be held constant while moving the tool past the point represented by that depth. This is to permit offsetting the instrument for correlation of the different measurements of the same formation. Unfortunately, this method of offset introduces added errors into the correlation between the measurements and into this time relationship with depth.
Typically, the conventional means of determining the length of cable lowered into the well has been the method of determining the depth of a logging tool within the well. Many devices have been proposed for measuring this length. Some of these are devices mechanically coupled to the sheave wheel while others resort to the use of detection devices responsive to magnetic marks on the cable or on the sheave wheel itself.
However, forces within the borehole act upon the logging device or the logging cable to cause changes in cable length which are not indicated by the surface measuring devices. Some of these forces include the weight of the logging tool and the weight of the cable connecting the tool to the reeling device at the surface of the earth and which cause a stretch in cable, positioning the logging tool at a location lower than indicated by the measuring instrument. Additionally, various forces within the well act on the logging instrument to slow it down. When this occurs in an upward traverse of the well, the cable begins to stretch and the instrument is situated again at a position different than indicated by the surface measuring device. As the instrument frees, it may override the position indicated by the surface indicator device and go through a series of oscillations termed in the art "yo-yo" until it is once again at the position approximately indicated by the surface measuring instruments. As an oil bearing strata may be two to five feet or less in thickness, the error introduced by such inaccurate depth indications can be excessive for an accurate determination of the location of such a formation when attempting to correlate the measurements obtained from different instruments.
Various prior art techniques have been developed to ascertain more accurately the position of a logging tool within the well at the time a measurement of a parameter of a subsurface formation was made and it depends for accuracy upon the simultaneous occurrence of the instrument making a measurement at precisely the depth indicated by the surface instrumentation. If, as is often the case in the prior art, the instrument was at a depth different than that indicated, attempts to move a second instrument in the string into position may result in erroneous data.
In summary, the foregoing problems of the prior art although attempting to provide technology for comparing logging data obtained of the same subsurface formation from different logging instruments have suffered limitations on the amount and quality of the data obtained which preclude accurate correlation of the various parameters obtained.
Accordingly, deficiencies in the prior art are overcome by the present invention wherein improved well logging methods and apparatus are provided whereby the depth and formation parameters obtained in such logging operations are correlated in a depth ranker than a time dependent relationship.